1. Field of the Invention
The present invention relates to a rare earth alloy sintered compact for use in, for example, an R—Fe—B based sintered magnet and a method of making such a sintered compact.
2. Description of the Related Art
A rare earth alloy sintered magnet (permanent magnet) is normally produced by compacting a powder of a rare earth alloy, sintering the resultant compact and then subjecting the sintered compact to an aging treatment. Permanent magnets currently used extensively in various fields of applications include a samarium-cobalt (Sm—Co) based magnet and a neodymium-iron-boron (Nd—Fe—B) based magnet. Among other things, an R—Fe—B based magnet (where R is at least one element selected from the rare earth elements including yttrium (Y) and is typically neodymium (Nd), Fe is iron and B is boron) is used more and more often in various types of electronic appliances. This is because an R—Fe—B based magnet exhibits a maximum energy product (BH)max that is higher than any of various other types of magnets and yet is relatively inexpensive.
An R—Fe—B based sintered magnet includes a main phase consisting essentially of a tetragonal R2Fe14B compound, an R-rich phase including Nd, for example, and a B-rich phase. In an R—Fe—B based sintered magnet, a portion of Fe may be replaced with a transition metal element such as Co or Ni and a portion of B may be replaced with C. An R—Fe—B based sintered magnet, to which various preferred embodiments of the present invention are applicable, is described in U.S. Pat. Nos. 4,770,723 and 4,792,368, for example.
In the prior art, an R—Fe—B based alloy has been prepared as a material for such a magnet by an ingot casting process. In an ingot casting process, normally, rare earth metal, electrolytic iron and ferroboron alloy as respective start materials are melted by an induction heating process, and then the melt obtained in this manner is cooled relatively slowly in a casting mold, thereby preparing an alloy ingot.
Recently, a rapid cooling process such as a strip casting process or a centrifugal casting process has attracted much attention in the art. In a rapid cooling process, a molten alloy is brought into contact with, and relatively rapidly cooled and solidified by, the outer or inner surface of a single chill roller or a twin chill roller, a rotating chill disk or a rotating cylindrical casting mold, thereby making a rapidly solidified alloy, thinner than an alloy ingot, from the molten alloy. The rapidly solidified alloy prepared in this manner will be herein referred to as an “alloy flake”. The alloy flake produced by such a rapid cooling process normally has a thickness of about 0.03 mm to about 10 mm. According to the rapid cooling process, the molten alloy starts to be solidified from a surface thereof that has been in contact with the surface of the chill roller. That surface of the molten alloy will be herein referred to as a “roller contact surface”. Thus, in the rapid cooling process, columnar crystals grow in the thickness direction from the roller contact surface. As a result, the rapidly solidified alloy, made by a strip casting process or any other rapid cooling process, has a structure including an R2Fe14B crystalline phase and an R-rich phase. The R2Fe14B crystalline phase usually has a minor-axis size of about 0.1 μm to about 100 μm and a major-axis size of about 5 μm to about 500 μm. On the other hand, the R-rich phase, which is a non-magnetic phase including a rare earth element R at a relatively high concentration, is dispersed in the grain boundary between the R2Fe14B crystalline phases.
Compared to an alloy made by the conventional ingot casting process or die casting process (such an alloy will be herein referred to as an “ingot alloy”), the rapidly solidified alloy has been cooled and solidified in a shorter time (i.e., at a cooling rate of about 102° C./sec to about 104° C./sec). Accordingly, the rapidly solidified alloy has a finer structure and a smaller average crystal grain size. In addition, in the rapidly solidified alloy, the grain boundary thereof has a greater area and the R-rich phase is dispersed broadly and thinly in the grain boundary. Thus, the rapidly solidified alloy also excels in the dispersiveness of the R-rich phase. Because the rapidly solidified alloy has the above-described advantageous features, a magnet with excellent magnetic properties can be made from the rapidly solidified alloy.
An alternative alloy preparation method called “Ca reduction process (or reduction diffusion process)” is also known in the art. This process includes the processing and manufacturing steps of: adding metal calcium (Ca) and calcium chloride (CaCl) to either the mixture of at least one rare earth oxide, iron powder, pure boron powder and at least one of ferroboron powder and boron oxide at a predetermined ratio or a mixture including an alloy powder or mixed oxide of these constituent elements at a predetermined ratio; subjecting the resultant mixture to a reduction diffusion treatment within an inert atmosphere; diluting the reactant obtained to make a slurry; and then treating the slurry with water. In this manner, a solid of an R—Fe—B based alloy can be obtained.
It should be noted that any block of a solid alloy will be herein referred to as an “alloy block”. The “alloy block” may be any of various forms of solid alloys that include not only solidified alloys obtained by cooling a melt of a material alloy either slowly or rapidly (e.g., an alloy ingot prepared by the conventional ingot casting process or an alloy flake prepared by a rapid cooling process such as a strip casting process) but also a solid alloy obtained by the Ca reduction process.
An alloy powder to be compacted is obtained by performing the processing and manufacturing steps of: coarsely pulverizing an alloy block in any of these forms by a hydrogen pulverization process, for example, and/or any of various mechanical grinding processes (e.g., using a ball mill or attritor); and finely pulverizing the resultant coarsely pulverized powder (with a mean particle size of about 10 μm to about 500 μm) by a dry pulverization process using a jet mill, for example. The alloy powder to be compacted preferably has a mean particle size of about 1.5 μm to about 7 μm to achieve sufficient magnetic properties. It should be noted that the “mean particle size” of a powder herein refers to a mass median diameter (MMD) unless stated otherwise.
An R—Fe—B based alloy powder is easily oxidizable, which is disadvantageous. A method of forming a thin oxide film on the surface of a rare earth alloy powder to avoid this problem was disclosed in Japanese Patent Gazette for Opposition No. 6-6728, which was originally filed by Sumitomo Special Metals Co., Ltd. on Jul. 24, 1986.
According to another known method, the surface of a rare earth alloy powder may also be coated with a lubricant for that purpose. It should be noted that a rare earth alloy powder with no oxide film or lubricant coating thereon, a rare earth alloy powder covered with an oxide film and a rare earth alloy powder coated with a lubricant will all be referred to as a “rare earth alloy powder” collectively for the sake of simplicity. However, when the “composition of a rare earth alloy powder” is in question, the composition is that of the rare earth alloy powder itself, not the combination of the powder and the oxide film or lubricant coating.
An R—Fe—B based sintered magnet produced by any of the methods described above does exhibit excellent magnetic properties. However, compared to a ferrite magnet, for example, a higher magnetizing field is needed to produce the R—Fe—B based sintered magnet. For example, when a motor including an R—Fe—B based sintered magnet is formed, a rare earth alloy sintered compact may be embedded in a portion of the motor and then magnetized by using a coil of the motor, for example (see Japanese Laid-Open Publication No. 11-113225, for example). In that situation, it is sometimes difficult to apply a magnetizing field with a sufficiently high strength to the sintered compact. An insufficiently magnetized magnet will exhibit inferior magnetic properties. Among other things, the remanence Br thereof may decrease considerably. In addition, such a magnet is easily demagnetized by heat, for example.
For example, Kanekiyo et al. described in Journal of Magnetics Society of Japan, Vol. 16, pp. 143–146 (1992) that the magnetization characteristic of an R—Fe—B based sintered magnet can be improved by adding Mo, V or Co to its material alloy.
Also, Japanese Laid-Open Publication No. 6-96928 discloses that the coercivity of an R—Fe—B based sintered magnet can be increased, and the demagnetization thereof can be decreased, by substituting Dy and/or Tb for a portion of Nd near the surface of an Nd2Fe14B intermetallic compound as a main phase.
However, the present inventors discovered and confirmed via experiments that other magnetic properties (the remanence Br, in particular) of the conventional magnets still decreased even when any of the above-described elements was added or substituted. Also, even if those other magnetic properties do not deteriorate, it is difficult to mass-produce the magnets because the elements to be added or substituted are rare and expensive.
In addition, it is known that if the average grain size of crystal grains that makes up a rare earth alloy sintered compact is decreased, the resultant magnet will exhibit increased coercivity. However, once the average crystal grain size is decreased, the magnetization characteristic of the sintered compact will deteriorate disadvantageously. Furthermore, once the particle size of the powder to be sintered is decreased, the powder becomes less easy to handle and shows a lower degree of orientation (i.e., degree of crystallographic orientation) during the compaction process.